Process for producing a surface acoustic wave device

ABSTRACT

This invention relates to a surface acoustic wave device and a production process thereof. An electrode is formed by alternately laminating a film of an aluminum alloy containing at least copper added thereto and a copper film on a piezoelectric substrate. While the particle size of the multi-layered electrode materials is kept small, the occurrence of voids in the film is prevented and life time of the surface acoustic wave device is elongated.

This application is a division of application Ser. No. 08/297,914, filedAug. 31 ,1994, now pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a surface acoustic wave device and aproduction process thereof.

Generally, a surface acoustic wave (SAW) device comprises apiezoelectric substrate and a comb-shaped interdigital electrodedisposed on the substrate, for converting a voltage to a surfaceacoustic wave or vice versa. The function of the surface acoustic wavedevice is to convert a radio frequency voltage to a surface acousticwave having a wavelength of about 10₋₅ times by using a comb-shapedinterdigital electrode, which causes this wave to propagate on thesurface of the piezoelectric substrate and converts again the wave tothe voltage by the comb-shaped interdigital electrode.

Frequency selectivity can be provided in accordance with the shape ofthe interdigital electrode during the two conversion operations betweenthe surface acoustic wave and the voltage, and a filter or a resonatorcan be constituted by utilizing this characteristic property. Becausethe propagation speed can be retarded to about 10₋₅ times that of anelectromagnetic wave, the surface acoustic wave device can be used as adelay device.

The application of the surface acoustic wave device to small, economicalfilters, resonators, delay lines, etc., has already been done byutilizing the functions described above. In other words, the surfaceacoustic wave device has been applied to IF filters of television sets,resonators of VTR (vide tape recorder) oscillators, VCOs of cordlesstelephones, and recently, the application has been expanded to RFfilters and IF filters of automobile telephones, mobile telephones, andso forth.

To further expand the utilization in this field, it is important toimprove a pass band and power characteristics of the surface acousticwave device. Particularly in the case of the automobile telephones andthe mobile telephones, transmission power is relatively great, that is,0.6 to 3W, and a large RF power is applied to a filter of a front-endportion inside the apparatus, particularly, to an antenna duplexer.

The maximum input power of the surface acoustic wave filter has beenabout 0.2W up to the present, and the filter lacks sufficient powercharacteristics. For this reason, a dielectric filter having high powerresistance has been used for the antenna duplexer. However, because thedielectric filter is large in scale, it causes a problem when the sizeof the apparatus is reduced as a whole.

Accordingly, if the power characteristics of the surface acoustic wavedevice can be improved and the antenna duplexer can be realized byutilizing the surface acoustic wave device, the mobile telephones can bemade even smaller, and the effect of utilization in industry becomesgreater.

2. Description of the Related Art

The interdigital electrode is used in the surface acoustic wave deviceas described above, and aluminum (Al) or an aluminum alloy containing asmall amount of a different kind of metal (not always a solid solutionbody in many cases) is generally used because the mass is small and itselectrical resistance value is low.

Several proposals have been made for the structure of the antennaduplexer using the surface acoustic wave device. Typical examples aredescribed in Japanese Unexamined Patent Publication (Kokai) Nos.5-167388 and 5-167389. In order to simplify the filter structure in theduplexer and to secure desired characteristics, Japanese UnexaminedPatent Publication (Kokai) No. 5-167388 proposes to constitute aduplexer by using a plurality of band-pass filters each formed by usingthe surface acoustic wave device. Japanese Unexamined Patent Publication(Kokai) No. 5-167389 proposes to integrate a plurality of surfaceacoustic wave band-pass filter chips having mutually different centerfrequency bands and having signal input/output terminals and groundterminals, by storing them in one package so as to minimize the duplexerwhile keeping excellent isolation.

However, the conventional antenna duplexer does not have characteristicssuch that the filer can sufficiently withstand the increase of RF power.To evaluate the power resistance or characteristics, the life time atthe maximum input power at which the apparatus can be used is generallyused as a guideline. The conventional antenna duplexer has a life timeof only about 1,600 hours at the 1W input at an environmentaltemperature of 85° C. (chip temperature of 120° C.) in an accelerateddeterioration test stipulated for the mobile telephones of the NTTspecification in Japan, for example. These values are not consideredsufficient for the life of mobile telephones, and the values of at leasttwice are believed necessary.

The main factor that determines the useful life of the surface acousticwave device is power characteristics of electrode fingers of the filter(interdigital electrode fingers IDT), and an aluminum system alloy filmcontaining a trace amount of copper and formed by sputtering, which iswell known as being resistant to migration in the field of semiconductordevices, has been used. However, this alloy is not yet sufficient as theelectrode material of the surface acoustic wave device used as theantenna duplexer to which a high power load is applied.

Besides the patent references described above, the following reportshave been made regarding the methods of improving electric power of theelectrode of the surface acoustic wave device.

1. Change of addition metal in aluminum (Al) system alloy:

The use of an aluminum-titanium alloy (AlTi), etc., for example, isdescribed in detail in "Examination of Al System Thin Film Material forSAW Power-Resistant Electrode and Production Method Thereof" (by Yuharaet al.), No. 17th EM Symposium Presume, pp. 7-12. According to thisreport, the useful life of the surface acoustic wave device can beimproved by about 10 times the life of an aluminum-copper (Al-Cu) alloyfilm by changing the electrode material to an aluminum-titanium (Al-Ti)alloy.

2. Use of aluminum (Al) epitaxial single crystal film:

This method is based on the fact that grain boundary diffusion in stressmigration of aluminum (Al) can be restricted by converting the structureto a single crystal, and is reported in papers of the Electronic DataCommunication Society, A, Vol. J76-A, No. 2, pp. 145-152 (1993) (by Iekiet al.). According to this report, life time can be improved to 2,000times that of an aluminum-copper alloy (Al-Cu) film by vacuumevaporation.

In comparison with films formed by sputtering, the useful life of analuminum-copper alloy (Al-Cu) film formed by vacuum evaporation is muchshorter from the beginning (refer to Yuhara et al., and otherreferences), and the improvement in life time is believed to besubstantially 20 to 200 times. At present, however, it has beenconfirmed only that this method can cause epitaxial growth only when thesubstrate material as the base is quartz, and cannot realize the filmwhen LiTaO₃ or LiNbO₃, which have been widely used as a substratematerial for filters for mobile communication, is employed.

As described above, stress migration in the surface acoustic wave deviceis analogous to electromigration and stress migration in wiringtechnology of semiconductor devices, and migration-resistant technologyin the semiconductor devices will be useful for the migration-resistanttechnology in the surface acoustic wave devices. Among them, thefollowing technology has drawn increasing attention.

Namely, it is the method which forms in a laminar form a film of anintermetallic compound of aluminum (Al) and a transition metal betweenthe aluminum (Al) films so as to block electromigration of the aluminum(Al) atoms by the intermetallic compound. This method is reported inU.S. Pat. No. 4,017,890 (J. K. Howard, IBM, Apr. 1977) and in connectionwith this patent, a report is made by J. K. Howard, J. F. White and P.S. Ho in "J. Appl. Phys., Vol. 49, p. 4083 (1978).

According to these reports, life time becomes maximal when chromium (Cr)is used as the transition metal, and is about 10 times that of thealuminum-copper alloy (Al-Cu). However, when the inventors of thepresent invention applied this method to the electrode of the surfaceacoustic wave device, a sufficient effect could not be obtained.

As described above, several methods have been proposed as the prior arttechnologies for improving the electrode materials, but none of themhave provided sufficient power characteristics. Accordingly, developmentof an electrode material having higher performance has been necessary.As a matter of fact, when the method of improving the powercharacteristics by the multi-layered structure of the aluminum films(Al) and the intermetallic compound of the aluminum (Al) and thetransition metal is applied to the surface acoustic wave device, noeffect can be observed but performance actually deteriorates.

FIG. 1 is an explanatory structural view of a surface acoustic wavefiler having the conventional three-layered structure. In the drawing,reference numeral 11 denotes a LiTaO₃ piezoelectric substrate, 12 is anAl-1%Cu alloy film, 13 is a Ta film, 14 is an Al-1%Cu film, and 15 and16 are Al-Ta alloy films.

In the surface acoustic wave filter using this conventionalthree-layered electrode structure, an 1,000Å-thick Al-1%Cu alloy film 12is formed on the LiTaO₃ piezoelectric substrate 11, a 500Å-thick Ta film13 is formed on the former, and a 1,000Å-thick Al-1%Cu film 14 isfurther formed on the Ta film 13. Next, heat-treatment is carried out at400° C. in vacuum so as to form sufficient Ta-Al (TaAl₃) 15 and 16 onthe interface between the Al-1%Cu films 12, 14 and the Ta film 13 and inthe grain boundaries of the Al-1%Cu alloy films 12, 14. The electrodestructure is then patterned into an interdigital shape to form theelectrode. When the useful life of this surface acoustic wave filter ismeasured by conducting an accelerated deterioration test at a chiptemperature of 120° C. and radio frequency power of 1W, the lifeexpectancy is found to be 100 hours, and drops to 1/16 of the life timeof a 3,200Å,-thick Al-1%Cu single layered film, that is, 1,600 hours.

FIG. 2 is a graph useful for explaining power characteristics of asurface acoustic wave filter having the conventional three-Alayeredstructure.

In the graph, the abscissa represents input power (W) and the ordinaterepresents life time (mean time to failure; MTTF, hours). Curve arepresents an Al-1%Cu single layer film which is not heat-treated, curveb represents an Al-1%Cu/Ta/Al-1%Cu film which is not heat-treated, andcurve c represents an Al-1%Cu/Ta/Al-1%Cu film which is heat-treated at400° C. The substrate (chip) temperature when forming each film is 120°C., and each film has a thickness of 3,200Å.

According to the J. K. Howard et al. reference described above, thesurface acoustic wave filter having the three-Alayered structureelectrode described above should provide longer life at least 20 timesthat of the Al-1%Cu film. According to experiments, however, the actuallife of the Al-1%Cu/Ta/Al-1%Cu film (see curve c) which is formed underthe ordinary heat-treatment conditions at 400° C. is much shorter thanthe life of the Al-1%Cu single layer film (see curve a) which is notheat-treated. This difference results from some differences of a lifedeterioration mechanism of wirings of semiconductor devices from a lifedeterioration mechanism of IDT (Interdigital Transducer) of the surfaceacoustic wave filter.

In short, both electromigration of the Al atoms and static stressmigration are involved in the life deterioration of the wirings of thesemiconductor devices, whereas the life deterioration of IDT of thesurface acoustic wave device mainly results from the dynamic stressmigration. Here, the static stress migration means the Al migrationdriven by the static internal stress of Al films. The dynamic stressmigration means the Al migration driven by the dynamic migration of theinternal stress caused by the acoustic surface wave propagation.Depending on parameters associated with the life deterioration, exactlyopposite actions result in some cases due to the difference ofelectromigration from the dynamic stress migration. A typical example isthe grain size of Al. According to J. B. Ghate,"Electro-migration-Induced Failure VLSI Interconnectors", Solid StateTechnology, pp. 113-120, 1983, the greater the grain size, the greaterthe effect of suppressing electromigration and the longer life becomes,in the case of the wirings of the semiconductor devices. On the otherhand, according to the afore-mentioned Yuhara et al. reference, thegreater the grain size, the shorter life becomes, in the case of thesurface acoustic wave device.

FIG. 3 is a graph useful for schematically explaining the relationbetween the grain size of the electrode material and life time. Theabscissa in the graph represents the grain size, and the ordinaterepresents life time. As shown in the graph, since electromigration ispredominant in the case of the wirings of the semiconductor device, lifetime becomes longer with the increase of the grain size (see curve b).In the case of the surface acoustic wave (SAW) device electrode, on theother hand, since stress migration is predominant, life time becomesshorter with the increase of the grain size of the electrode material(see curve a). The grain size of the electrode material can be increasedby applying heat-treatment.

It can be interpreted from the sequence described above that the causeof deterioration of the Al-1%Cu/Ta/Al-1%Cu film formed conventionally byapplying heat-treatment at 400° C. and represented by the curve c inFIG. 2 is this heat-treatment at 400° C., since the grain size becomesgreater and stress migration becomes more likely to occur due to thisheat-treatment, so life time is reduced. To further support this fact, athree-layered film having the same structure is formed without carryingout the heat-treatment and moreover, in such a manner that thetemperature never exceeds 200° C. throughout the full process, so as toconstitute the surface acoustic wave filter. When life of this filter isevaluated, the curve b in FIG. 2 can be obtained, and life time issubstantially equal to that of the Al-1%Cu single layered film (seecurve a).

It can be understood that when the heat-treatment is not carried out ata high temperature of about 400° C., life time can be drasticallyimproved. This is because the grain size can be kept small. In thiscase, although the grain size remains small and life time is relativelylong, the alloy between Al and the transition metal is not formedbetween the layers because heat-treatment is not effected, and becausethe function of a stopper for inhibiting cracks occurring in the film,that is, the growth of voids, does not exist, so life time is notimproved in comparison with the Al-1%Cu single layered film which is notheat-treated (see curve a).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a surface acousticwave device which can prevent the occurrence of voids in a film whilekeeping a grain size of an Al-Cu multi-layered film small, and which hasa long life time.

It is another object of the present invention to provide a process forproducing such a surface acoustic wave device.

These and other objects of the present invention will become moreapparent from the following detailed description of preferredembodiments thereof.

According to the present invention, there is provided a surface acousticwave device which comprises a piezoelectric substrate and an electrodeformed on the substrate by alternately laminating a film of aluminumcontaining at least copper added thereto or an alloy of such aluminumand a copper film. In this case, the electrode is a transducer forconverting an electrical signal to a surface acoustic wave.

In the surface acoustic wave device according to the present invention,directions of internal stresses of the film of aluminum containing atleast copper or the alloy of such aluminum and the copper filmpreferably have opposite directions, and moreover, the sum of theseinternal stresses are zero (0) or compressive (stress on the negativeside). When the internal stresses are regulated in this way, stressmigration of aluminum can be reduced.

A laminate structure of the aluminum or aluminum alloy film/copper filmconstituting the electrode can be constituted arbitrarily into a two- ormore multi-layered structure, and is preferably a two- or three-layeredlaminate structure. In such a multi-layered structure, the thickness ofeach film can be broadly changed in accordance with frequency and othervarious factors, but is generally and preferably within the range offrom about 300Åto about 10,000Å.

In a preferred embodiment of the present invention, the electrode can bea two-layered laminate structure of the aluminum-copper alloy film andthe copper film. Here, the thickness of the Al-Cu film for 800 to 1,000MHz filters is preferably from about 1,000Åto about 5,000Å, and thethickness of the Cu film is preferably from about 300 to about 1,000Å.

In another preferred embodiment of the present invention, the electrodecan be a three-layered laminate structure comprising two aluminum-copperalloy films and the copper film sandwiched between the aluminum-copperalloy films. The thickness of each of the Al-Cu films for 800 to 1,000MHz filters is preferably from about 500Åto about 1,500Å, and thethickness of the Cu film is preferably from about 300Åto about 1,000Å.

In the surface acoustic wave device according to the present invention,it is essentially necessary to add copper to the aluminum or aluminumalloy film constituting the electrode. The amount of addition of copperis preferably from 0.4 to 4 wt% and further preferably, from 0.5 to 1.5wt% on the basis of the weight of the film. If the amount of addition ofcopper is below 0.4 wt%, problems such as stress migrations appear, andif it exceeds 4 wt%, on the other hand, fine patterns of IDT can not bedelineated by RIE (reactive ion etching) because of copper-basedresidue.

In the embodiments of the present invention, copper is most preferablyadded to the aluminum or aluminum alloy film.

The piezoelectric substrate used as the substrate can be thosepiezoelectric crystal substrates which are ordinarily used in surfaceacoustic wave devices, such as LiNbO₃, LiTaO₃, quartz, ZnO/glass, PZTtype ceramics, and so forth. Preferably, LiTaO₃, such as (36°Y-X)LiTaO₃and LiNbO₃ such as (64°Y-X)LiNbO₃, can be used effectively as thepiezoelectric substrate.

When a surface acoustic wave device having a piezoelectric substrate andan electrode formed on the substrate is produced, the present inventionprovides a process for producing a surface acoustic wave device whichcomprises alternately laminating a film of aluminum containing at leastcopper added thereto and a copper film on the piezoelectric substrate ata temperature not higher than 200° C.; patterning the resulting laminatestructure to form an electrode; and carrying out subsequent processingswhile maintaining the temperature of not higher than 200° C.

The piezoelectric substrate and the electrode formed on the substratehave already been described above. The electrode can be formed bylaminating the respective films into a predetermined film thickness byordinary film formation technology such as sputtering, CVD (ChemicalVapor Deposition), electron beam deposition, etc., and subsequentlypatterning the resulting laminate structure into a desired electrodeshape.

The method of the present invention can restrict the growth of the grainboundary of the electrode materials by employing the process stepsdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view useful for explaining the structure of asurface acoustic wave filter having a three-layered structure accordingto the prior art;

FIG. 2 is a graph useful for explaining power characteristics of asurface acoustic wave filter having a three-layered structure accordingto the prior art;

FIG. 3 is a graph useful for explaining the relationship between a grainsize of an electrode material and life time;

FIG. 4 is a perspective view useful for explaining the structure of asurface acoustic wave device according to one embodiment of the presentinvention;

FIG. 5 is a graph useful for explaining experimental results of a filmthickness and an internal stress of each of an Al-1%Cu film and a Cufilm;

FIG. 6 is a perspective view useful for explaining the structure of asurface acoustic wave filter according to an embodiment of the presentinvention;

FIG. 7 is an equivalent circuit diagram of the surface acoustic wavefilter shown in FIG. 6;

FIG. 8 is a graph showing transmission characteristics of the surfaceacoustic wave filter according to an embodiment of the presentinvention;

FIG. 9 is a graph showing power characteristics of the surface acousticwave filter according to an embodiment of the present invention;

FIG. 10 is an explanatory view of an Al-Cu film electrode structure;

FIG. 11 is a perspective view useful for explaining an Al-Cu/Cu filmelectrode structure;

FIG. 12 is a schematic view useful for explaining a CuAl₂ crystalstructure;

FIG. 13 is a perspective view useful for explaining an Al-Cu/Cu/Al-Cufilm electrode structure; and

FIG. 14 is a graph showing the relationship between an internal stressof an alloy film and power characteristics of a surface acoustic wavedevice.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, an electrode structure which can be advantageously utilized in asurface acoustic wave device according to the present invention, and thefunction and effect of such an electrode, will be explained.

Generally, it is believed that a film obtained by adding a small amount(about 3 to 4 wt%) of a different kind of metal to Al has a structure inwhich an alloy between Al and the different kind of metal exists at agrain boundary of Al.

FIG. 10 is an explanatory view of an Al-Cu film electrode structure. Inthis drawing, reference numeral 21 denotes an LiTaO₃ substrate, 22 is anAl-Cu film, 23 is Al crystal grains, 24 is a grain boundary, and 25 isCuAl₂. This drawing illustrates an example where the Al-Cu film 22 isdeposited on the LiTaO₃ substrate 21 by sputtering or electron beamdeposition and is patterned. Basically, it is a polycrystallinestructure of Al, wherein a large number of Al crystal grains 23 exist,and CuAl₂ 25 segregates at the grain boundary 24. It is believed thatthe reason why the Al-Cu film has higher resistance to migration thanthe Al film is because Cul2 25 inhibits fluidization of the Al atoms.

A similar effect can be obtained when Ti, Si, etc., is used as the metalto be added to Al, in place of Cu described above.

Next, we consider the case where a Cu film is formed on the uppersurface of the Al film having this structure, with reference to FIG. 11.

FIG. 11 is an explanatory view of an Al-Cu/Cu film electrode structure.In the drawing, reference numeral 21 denotes a LiTaO₃ substrate, 22 isan Al-Cu film, 23 is Al crystal grains, 24 is a grain boundary, 25 isCuAl₂ and 26 is a Cu film. The drawing illustrates an example where theAl-Cu alloy film 22 is formed on the LiTaO₃ substrate 21 by sputteringor electron beam deposition, the Cu film 26 is formed on the former andthe Cu film 26 is then patterned. This is basically a polycrystallinestructure of Al. A large number of Al crystal grains 23 exist, and CuAl₂25 segregates between the grain boundary 24, the Al-Cu film 22 and theCu film 26.

Even when the temperature is as low as below 200° C. when forming the Cufilm 26 on the Al-Cu film 22, a small amount of CuAl₂ 25 is formed onthe interface between the grain boundary, the Al-Cu film 22 and the Cufilm 26. The reason for this is believed to be as follows. Cu to besputtered has large kinetic energy and impinges against Al, and the filmis formed while Cu imparts kinetic energy to the Al atoms. Therefore, aneffect similar to the effect of local heat-treatment occurs, and CuAl₂is formed on the interface between the Al-Cu alloy film 22 and the Cufilm 26. The thickness of Cu-Al₂ on the interface is some dozens ofangstroms (Å).

Now, let's consider the case where the Al-Cu alloy film is furtherformed on the film having the structure shown in FIG. 11.

FIG. 12 is an explanatory view of the CuAl₂ crystal structure. As shownin the drawing, the CuAl₂ crystal has the structure wherein the Culayers and the Al layers are alternately laminated. Therefore, matchingwith the Cu film is extremely excellent, and firm bonding can beexpected. Because CuAl₂ 25 existing in the Al grain boundary 24 of theAl-Cu film 22 shown in FIG. 11 and CuAl₂ 25 existing on the interfacebetween the Al-Cu film 22 and the Cu film 26 are the same crystal,mutual bonding strength becomes high.

FIG. 13 is an explanatory view of an Al-Cu/Cu/Al-Cu film electrodestructure. In this drawing, reference numeral 21 denotes a LiTaO₃substrate, 22 is an Al-Cu film, 23 is an Al crystal grain, 24 is a grainboundary, 25 is CuAl₂, 26 is a Cu film, 27 is an Al-Cu film, 28 is an Alcrystal grain, 29 is a grain boundary, and 30 is CUl₂. The drawingillustrates an example where the Al-Cu film 22 is formed on the LiTaO₃substrate 21 by sputtering or electron beam deposition, the Cu film 26is formed on the former, and the Al-CU film 27 is further formed on theCu film 26 and is patterned. CuAl₂ 25 is formed in the grain boundary 24of the Al crystal grains 23 of the Al-Cu film 22,CuAl₂ 30 is formed inthe grain boundary 29 of the Al crystal grains 28 of the Al-Cu film 27,and CuAl₂ is further formed between the Cu film 26 and the upper andlower Al-Cu films 22, 27.

Under such a condition,CuAl₂ 25, 30 existing in the grain boundaries 24,29 in the upper and lower Al-Cu films 22, 27 and CuAl₂ existing on theinterface between the Al-Cu films 22, 27 and the Cu film 26 are stronglybonded to one another, and the Cu film at the center of the film as awhole functions as the framework, while CuAl₂ existing in the grainboundaries of the upper and lower Al-Cu film has a small bone networkstructure. Accordingly, a film having high resistance to stressmigration can be realized at a low temperature of below 200° C. Whenheat-treatment is applied to the film, CuAl₂ on the interface becomesthick, but because the Al crystal grains grow to a large grain size asdescribed already, the resistance to stress migration drops.Accordingly, heat-treatment at a high temperature above 200° must not beapplied.

As described above, the fundamental principle of the present inventionlies in that the Al-Cu film and the Cu film are laminated, and thenetwork structure is formed by CuAl₂ formed in the grain boundary of Alin the Al-Cu film with the Cu film being the center, so as to inhibitstress migration.

As described in the afore-mentioned Yuhara et al. reference, also, thefundamental principle of the present invention is based on the conceptthat the internal stress of the Al alloy film is largely associated withpower characteristics (life) of the surface acoustic wave device, powercharacteristics are high when the stress of the Al alloy film is zero orrather compressive, and power characteristics drop with higher tensilestress.

FIG. 14 is a graph showing the relation between the internal stress ofthe alloy film and power characteristics of the surface acoustic wavedevice. This graph cites the data reported previously by Yuhara et al.The axis of abscissa represents the internal stress of the alloy film,and the ordinate represents the stress of the surface acoustic wavedevice, that is, the tendency of power characteristics. As can be seenfrom this graph, power characteristics of the surface acoustic wavedevice are high when the internal stress of the alloy film is zero orcompressive, but are deteriorated when the internal stress is tensile.

Accordingly, power characteristics can be improved by arranging thefilms so that their internal stresses have opposite signals when themulti-layered alloy film is formed, and moreover, the magnitudes of theinternal stresses are mutually in equilibrium, in order to regulate theinternal stress of the film as a whose to zero or somewhat compressive.

Next, several embodiments of the present invention will be explainedwith reference to the drawings. It is to be understood that theseembodiments are merely illustrative and in no way limit the presentinvention.

FIG. 4 is an explanatory structural view of a surface acoustic wavedevice according to an embodiment of the present invention. In thedrawing, reference numeral 1 denotes a LiTaO₃ substrate, 2 is an Al-1%Cufilm, 3 is Al crystal grains, 4 is a grain boundary, 5 is CuAl₂, 6 is aCu film, 7 is an Al-1%Cu film, 8 is Al crystal grains, 9 is a grainboundary, and 10 is CuAl₂. In the surface acoustic wave device of thisembodiment, a 1,000Å -thick Al-1%Cu film 2 is formed on the LiTaO₃substrate 1 having a piezoelectric property while the temperature iskept below 200° C., a 400Å -thick Cu film 6 is formed on the former, anda 1,000Å -thick Al-1%Cu film 7 is formed on the Cu film 6. In this way,a three-layered film having a total thickness of 2,400Å is formed. Thisthree-layered laminate film is patterned to form an interdigitalelectrode (hereinafter referred to as the "three layered film electrodeA").

In the embodiment shown in the drawing, CuAl₂ 5 is formed in the grainboundary 4 of the Al crystal grains 3 of the Al1%Cu film 2, CUAl₂ 10 isformed in the grain boundary 9 of the Al crystal grains 8 of the Al1%Cufilm, and CuAl₂ 5, 10 is also formed between the Cu film 6 and the upperand lower Al1%Cu films 2, 7.

To compare with the three-layered electrode A of this embodiment, aninterdigital electrode consisting of a 3,200Å -thick Al1%CUsingle-layered film (hereinafter referred to as the "single-layered filmelectrode C") is formed on the LiTaO₃ substrate.

To compare the effect of stress regulation of the three-layered filmelectrode, an interdigital electrode (hereinafter referred to as the"three-layered film electrode B") is formed by first forming a 700Å-thick Al-1%Cu film, a 600Å -thick Cu film and a 700Å -thick Al-1%Cufilm on the LiTaO₃ substrate in the total thickness of 2,000Å andpatterning this three-layered laminate film.

To examine the heat-treatment effect of the three-layered film electrodeA, an interdigital electrode (hereinafter referred to the "three-layeredfilm electrode A") is formed by heat-treating the three-layered filmelectrode A at 400° C. after the film formation.

The thickness of these electrode films is determined in the followingway.

A. As a reference a 3,200Å -thick Al1%CU single layer film will beconsidered.

When a surface acoustic wave filter is produced using this Al1%Cu singlelayer film as the electrode by the later-appearing method, atransmission band-pass filter of an NTT specification having 933 MHz asthe center frequency can be realized.

In the surface acoustic wave filter, the center frequency changes inaccordance with the mass of the electrode due to the mass load effect.Therefore, in order to correctly compare power characteristics when theelectrode is changed, it is necessary to bring the mass of the electrodefilm into conformity with the mass of the electrode of the surfaceacoustic wave filter using the Al1%Cu single layer film electrode C soas to prevent frequency fluctuation.

The density of Cu is 8.9, the density of l is 2.7, and the density ofthe Cu film is about three times the density of Al. Therefore, themasses of the three-layered film electrodes A, B and AA aresubstantially the same as the mass of the 3,200Å -thick Al1%Cu singlelayer film electrode as the reference. Accordingly, the surface acousticwave filters using the three-layered film electrodes A, B and AA exhibitsubstantially the same characteristics as the 933 MHz filter.

B. The balance of the internal stresses of the multi-layered filmelectrode must be secured so as to improve power characteristics asalready described.

If the substrate temperature and the film formation rate at the time ofgrowth of the multi-layered film are constant, the internal stress ofthe multi-layered film depends on the film thickness of each layer.

FIG. 5 is a graph useful for explaining the experimental results of theinternal stresses of the Al-1%Cu film and the Cu film. In this graph,the abscissa represents the film thickness of the metal film, and theordinate represents the stress. In the graph, the experimental resultsof the film thickness of the Al-1%Cu film and the Cu film, and theinternal stress are plotted.

When the balance of the internal stress inside the laminate film istaken into consideration, the stress is -6×10⁸ N/m² (the-sign representsthe compressive stress and the+sign represents the tensile stress) inthe case of the Cu film at a thickness of 400Å , and +2×10⁸ N/M² in thecase of the Al-1%Cu film at a film thickness of 1,000Å in thethree-layered film electrode A consisting of the Al-1%Cu film/Cufilm/Al-1%Cu film. Therefore, the stress is -2×10⁸ N/m² in thethree-layered film electrode as a whole, and a weak compressive stressis applied. According to FIG. 14 previously explained, this internalstress -2×10⁸ N/M² is included in a region in which powercharacteristics of the multi-layered film electrode are notdeteriorated.

In the case of the three-layered film electrode B, the stress value is-1×10⁸ N/m² for the Cu film at a thickness of 600Å, and 2×2.5×10⁸ N/M²for each Al-1%Cu film at a thickness of 700Å. The total stress is 4×10⁸N/M², and is the tensile stress. According to FIG. 14, this internalstress of 4×108⁸ N/M² is included in the region where powercharacteristics of the multi-layered film electrode are deteriorated.

FIGS. 6 and 7 are explanatory structural views of the surface acousticwave filter according to one embodiment of the present invention,wherein FIG. 6 is a perspective view and FIG. 7 is an equivalent circuitdiagram. In the drawings, symbol T_(in) denotes an input terminal,T_(out) is an output terminal, R_(p1) is a first parallel resonator,R_(p2) is a second parallel resonator, Rp_(p3) is a third parallelresonator, R_(s1) is a first series resonator, R_(s2) is a second seriesresonator, and R_(p11), RP_(p12), R_(p21), R_(p22), R_(p31), R_(s32),R_(s11), R_(s12), R_(s21) and R_(s22) are reflectors.

The surface acoustic wave filter according to this embodiment isdescribed in detail in Japanese Unexamined Patent Publication (Kokai)No. 5-183380 to which reference is hereby made. The multi-layered filminterdigital electrode of this embodiment is formed on a 36°Y-X LiTaO₃piezoelectric substrate of 1.5×2×0.5 mm, and the first series resonatorR_(s1) and the second series resonator R_(s2) are connected in seriesfrom the input terminal T_(in) towards the output terminal T_(out). Thefirst, second and third parallel resonators R_(p1), R_(p2) and R_(p3)are grounded from the junction between the input terminal and the firstseries resonator R_(p1), from the junction between the first and secondseries resonators R_(s1), R_(s2) and from the junction between thesecond series resonator R_(s2) and the output terminal.

The reflectors R_(s11), R_(s12) are provided to the first seriesresonator R_(s1), and the reflectors R_(s21), Rs_(s22) are provided tothe second series resonator R_(s2). The reflectors R_(p11), R_(p12) areprovided to the first parallel resonator R_(p1), and the reflectorsR_(p21), R_(p22) are provided to the second parallel resonator R_(p2).Further, the reflectors R_(p3), R_(p32) are provided to the thirdparallel resonator R_(p3).

The 0.5 mm-thick LiTaO₃ piezoelectric substrate is used in such a mannerthat its 1.5 mm side as the x-axis direction of the crystal axis existsin the transverse direction of the drawing and its 2 mm side exists inthe longitudinal direction of the drawing, or in other words, in thepropagating direction of the surface acoustic wave. The pitch λ_(p) ofthe electrodes of the first parallel resonator R_(p1), is set to 4.39μm,its aperture length is set to 160μm, the aperture length of the firstseries resonator R_(s1) is set to 60μm, and the electrode pitch of thesecond series resonator R_(s2) is set to 4.16μm.

FIG. 8 is a graph showing the transmission characteristics of thesurface acoustic wave filter according to one embodiment of the presentinvention. The abscissa in the graph represents frequency (MHz) and theordinates represents attenuation (dB). As shown in the graph, thesurface acoustic wave filter has the characteristics of a band-passfilter having an about 60 MHz pass band in the proximity of 930 MHz.Attenuation in the pass band is 1.5 dB.

The life test of this surface acoustic wave filter is carried out byselecting a frequency, at which power characteristics are the lowestamong the pass band, that is, near 950 MHz in this embodiment, andapplying a radio frequency power thereto. At this time, the temperatureof the filter chip rises somewhat, but an external temperature iscontrolled in taking such a temperature rise into consideration inadvance, and radio frequency power and its life are controlled while thesurface temperature of the filter chip is kept constant.

FIG. 9 is a graph showing the power characteristics of the surfaceacoustic wave filter according to one embodiment of the presentinvention. The abscissa in the graph represents input power (W) and theordinate represents mean time to failure (MTTF). The failure is definedby the degradation of 0.3 dB for 1.5 dB insertion loss in the pass band(see, FIG. 8).

Generally, when the MTTF of the electrode of the surface acoustic wavefilter relies on the Arrhenius'equation, that is,

    1n(MTTF) =A+B/T-n×1n(Pin),

the natural logarithm of the input power (Pin) and the natural logarithmof mean time to failure (MTTF) are expressed by rightwardly descendingstraight lines. Here, A, B and n are proportional constants.

Besides the surface acoustic wave filter using the electrode of thisembodiment, this FIG. 9 shows also the life time of the surface acousticwave filters using the four kinds of the electrodes described above,respectively. In this measurement, the filter chip temperature T is setto 393 K (120° C.).

Curve a in FIG. 9 represents the life time of the surface acoustic wavefilter using the Al1%Cu/Cu/l-1%Cu film (three-layered film electrode A)which is not heat-treated and has a compressive stress of -2×10⁸ N/m².Curve b represents the life time of the surface acoustic wave filterusing the Al1%Cu/Cu/l-1%Cu film (three-layered film electrode B) whichis not heat-treated and has a tensile stress of +4×10⁸ N/M². Curve crepresents the life time of the surface acoustic wave filter using theAl1%Cu film (single layer film electrode C) which is not heat-treated,and curve d represents the life time of the surface acoustic wave filterusing the Al1%Cu/Cu/Al-1%Cu film (three-layered electrode AA) which isheat-treated at 400° C. By the way, the substrate temperature whenforming each film is 120° C.

In comparison with the surface acoustic wave filter (see curve c) usingthe conventional Al1%Cu single layer film (single layer film electrodeC), the life time of the surface acoustic wave filter (see curve a)using the l-1%Cu/Cu/Al-1%Cu film (three-layered film electrode A) ofthis embodiment, which is not heat-treated and has the compressivestress of -2×10⁸ N/M² is 120 times.

The life time of the acoustic wave filter (see curve d) having thethree-layered film (three-layered film electrode AA) obtained byheat-treating the Al-1%Cu/Cu/Al-1%Cu film (three-layered electrode A) ofthis embodiment which is not heat-treated and has a compressive stressof -2×10⁸ N/M², becomes drastically short, and is shorter than the lifetime of the surface acoustic wave filter (see curve c) using theconventional Al-1%Cu single film layer (single layer film electrode).

Further, the life time of the Al1%Cu/Cu/Al-1%Cu film (three-layered filmelectrode B) which is not heat-treated and has a tensile stress of+4×10⁸ N/M² (see curve b) is improved in comparison with the life timeof the surface acoustic wave filter (see curve c) using the conventionalAl1%Cu single layer film (single layer film electrode C), but isincomparatively shorter than the life time of the surface acoustic wavefilter of this embodiment having the internal stress thereof regulated(see curve a).

The surface acoustic wave filter of this embodiment can provide 200,000hours as the useful life at the time of input of 1W. Accordingly, thefilter can be said to have sufficient power characteristics as anantenna duplexer.

Though a general piezoelectric crystal substrate can be used as thepiezoelectric substrate, the piezoelectric materials illustrated in thisembodiment, such as LiTaO₃ (36° Y cut-X propagation), LiNbO₃ (64° Ycut-X propagation), etc., are effective in order to improve thecharacteristics of the filter, and the like.

As described above, the present invention employs the multi-layeredstructure of the Al-Cu film/Cu film/Al-Cu film as the electrodematerial. Therefore, even in the case of surface acoustic wave deviceswhich cannot be heat-treated at a high temperature due to stressmigration, the present invention can drastically improve their powercharacteristics, and greatly contributes to the improvement inperformance of the surface acoustic wave devices such as the surfaceacoustic wave filters.

We claim:
 1. A process for producing a surface acoustic wave devicehaving a piezoelectric substrate and an electrode disposed on saidsubstrate, comprising the steps of:alternately laminating an aluminumcopper alloy film and a copper film on said piezoelectric substrate at atemperature not higher than 200° C. to thereby form a laminate structurehaving at least three layers, with two aluminum-copper alloy filmssandwiching one copper film; patterning the resultant laminate structureto form an electrode; and carrying out subsequent processings whilemaintaining the temperature of not higher than 200° C.
 2. A process forproducing a surface acoustic wave device according to claim 1, whereinthe laminate structure consists of two aluminum-copper alloy filmssandwiching one copper film.
 3. A process for producing a surfaceacoustic wave device according to claim 1, wherein the piezoelectricsubstrate is made of a piezoelectric material selected from the groupconsisting of LiTaO₃ and LiNbO₃ .
 4. A process for producing a surfaceacoustic wave device according to claim 1, wherein:the aluminum-copperalloy films have one of a tensile internal stress and a compressiveinternal stress, the copper film has the other of a tensile internalstress and a compressive internal stress, such that the internalstresses of the aluminum copper alloy films and the cooper film havemutually opposite directions, and the sum of the internal stresses iseither zero or compressive.
 5. A process for producing a surfaceacoustic wave device according to claim 1, wherein the aluminum-copperalloy films are polycrystalline films having aluminum crystal grains andCuAl₂ segregated at a grain boundary thereof.
 6. A process for producinga surface acoustic wave device according to claim 1, wherein thealuminum-copper alloy films are formed by sputtering or electron beamdeposition.